Elemental analysis is a method for the determination of carbon, nitrogen, hydrogen, oxygen and/or sulphur composition of different materials, including liquids, solids and gases. During elemental analysis, samples are typically converted to simple gases such as H2, CO, CO2, N2, SO2, and H2O, usually by combustion or reduction/pyrolysis in a high temperature reactor (usually at about or exceeding 1000° C.), and usually with aid of catalysts to facilitate the combustion. Also a combination of two or more reactors are not uncommon, e.g. one oxidation reactor combined with a reduction reactor to reduce e.g. nitric oxides to nitrogen. The combustion products are carried by an inert carrier gas (e.g. He or Ar) to a detector. The samples can subsequently be transported for further detection or simply vented. To allow quantitative or qualitative determination of each gas species, the mixture is separated, for example in one or more chromatographic columns, such as a gas chromatographic column, or by adsorption/thermodesorption techniques, and detected using for example flame photometric detection, atomic absorption spectroscopy, inductively coupled plasma optical emission spectrometry (ICP-OES), optical absorption spectrometers (e.g. for infrared absorption), mass spectrometers, including inductively coupled plasma mass spectrometry (ICP-MS), glow discharge mass spectrometry (GD-MS), or by mass spectrometers for isotope ratio analysis.
Typical systems comprise a reactor to convert sample material to simple gases, one or more chemical traps to adsorb undesired gas analytes such as H2O, one or more separation columns and a detector. For high reproducibility, the system is permanently flushed with the carrier gas. This is done to maintain pressure and temperature regimes, to avoid introduction of contaminant gases such as air, and to avoid damage to any materials and chemicals within the system and to continuously flush away contamination. Elemental analyzers can require a flow up to, or in excess of, 1000 mL/min, which varies depending on the volume that needs to be flushed. The biggest volume in the system is the combustion and/or reduction or pyrolysis reactor.
In some commonly used systems, a flow rate of 40 to 300 mL/min, or more commonly 80 to 200 mL/min is required. Typical analysis time of a sample is up to 15 minutes. Of this time, the analyte gases pass through the reactor in less than 3 minutes, and for the remaining time, the system is flushed with carrier gas which is usually vented and thus wasted to atmosphere.
The most common carrier gas is helium, but in recent times this gas has become expensive, in part due to less availability. To reduce cost, it is common to reduce helium consumption in elemental analyzers by argon, for example in systems provided by Thermo Fisher Scientific S.p.A (Rodano, Italy), Elementar Analysensysteme (Hanau, Germany), Perkin Elmer (Waltham, Mass., USA) and LECO Corporation (St. Joseph, Mich., USA). However, argon suffers from the disadvantage that it has a high thermal conductivity and high ionization efficiency, which make it particularly unfavourable when a mass spectrometer is used for detection. Additionally, use of argon as carrier gas requires recalibration of volume flow controller and replacing detectors, since current detectors and applications of the systems are adapted to use of helium as carrier gas.
Further helium consumption is needed for purposes of a reference signal in the thermal conductivity detector (TCD) of a typical elemental analyzer. This reference flow of pure helium compares the thermal conductivity of the reference with the mixture of helium and analyte gas that arrives at the detector. The reference flow is also used to purge the sample in the sample injection system (for example autosamplers) before it is injected into the reactor, to remove ambient air. This reference/purge flow of carrier gas thus prevents contaminating gases such as nitrogen oxygen, and water from entering the reactor during injection. The helium purge flow can be as high as 250 mL/min, and is therefore a major source of helium consumption.
WO 98/36815 discloses a carrier gas recycling system for an analytical instruments such as a gas chromatograph, that collects, purifies, compresses and recycles carrier gas. The system includes means to collect a carrier gas along with contaminants, a purifier to remove contaminants from the carrier gas, a compressor to compress the collected and purified gas, a source of a make-up carrier gas, and means to receive recycled, purified gas and introduce the recycled carrier gas into the analytical instrument.
U.S. Pat. No. 6,293,995 discloses a gas chromatograph that includes a closed-loop system for storage and reuse of hydrogen carrier gas. The chromatograph includes a gas storage system that receives a gas output from a detector, and stores the gas for subsequent reuse. The storage system preferably includes a metal hydride storage system.
In U.S. Pat. No. 8,308,854, a system for recycling helium gas is disclosed, that includes a bladder to receive helium-bearing gas from a vent of a gas chromatograph, a source of pressurized air or gas to supply gas into a compartment containing the bladder so as to compress the bladder containing the helium bearing gas, a gas reservoir coupled to the bladder interior so as to receive the helium bearing gas, and at least one purification module for removing contaminants from the helium bearing gas, and an output for coupling the purification module to a carrier gas inlet of the gas chromatograph.
The present invention has been made against this background, to provide an improved and flexible system that consumes less carrier gas than systems previously described in the art. The invention further provides a method of elemental analysis that requires reduced gas consumption.